In industrial sites, hydrogen production is conducted in a manner such that natural gas, naphtha, and other hydrocarbon sources are subjected to steam reforming or such that hydrogen is recovered from various gas mixtures of petrochemical industries. Conventionally, hydrogen purification is mainly realized through a PSA process using discharge gas from a steam methane reformer.
In a typical PSA process, components of a gas mixture are separated using differences in adsorptive selectivity of adsorbates to an adsorbent. The adsorption procedure for separating a less strongly adsorbable component from a more strongly adsorbable component is chiefly conducted at high pressure, and the pressure of the adsorption column is decreased so that the adsorbed component is desorbed, thus regenerating the adsorbent.
In order to sufficiently regenerate the adsorbent, a purging step is conducted at low pressure using a less strongly adsorbable component having high purity and, for pressurization to adsorption pressure, a feed gas or a hydrogen product is used.
The PSA method for hydrogen purification is conducted through a process composed principally of an adsorption step, a countercurrent depressurization step, a purging step, and a pressurization step. However, when only this process is applied, the recovery of mechanical energy to the adsorption unit is also decreased, as well as the recovery of a hydrogen product.
The adsorption step is conducted until the purity of a hydrogen product is maintained as desired under high pressure. After the completion of the adsorption step, the pores of the adsorption column are packed with a less strongly adsorbable component having a concentration not lower than that of feed gas. So, a countercurrent depressurization step is conducted immediately after the completion of the adsorption step at high pressure, thus regenerating the adsorbent.
However, in the course of regeneration of the adsorbent, the less strongly adsorbable component, for example, hydrogen or the like, is lost, undesirably decreasing the recovery of the less strongly adsorbable component. Therefore, with the goal of increasing the recovery of the less strongly adsorbable component, a cocurrent depressurization step or a pressure equalization step is introduced.
The gas, which is discharged in the cocurrent depressurization step or the pressure equalization step, is used to pressurize another adsorption column during a pressurization step and to purge a further adsorption column during a purging step, thereby maintaining mechanical energy in the adsorption unit and contributing to an increase in the recovery and purity of hydrogen products.
Generally, a gas mixture, which is a feed gas to be supplied to a hydrogen purification process, includes impurities such as carbon dioxide, nitrogen, methane, water, and carbon monoxide. Among these impurities, carbon monoxide is problematic in that it is very difficult to remove.
The prior patents disclosing the PSA method for hydrogen purification, as noted above, are briefly described below.
U.S. Pat. No. 3,430,418 discloses a PSA process for purifying hydrogen from a hydrogen-containing gas mixture including impurities such as carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), nitrogen (N2), and water (H2O) using two adsorbents. In this disclosure, the activated carbon adsorbent is used to remove methane (CH4), carbon dioxide (CO2), and water (H2O), and calcium (Ca) ion-exchanged zeolite 5A is used to remove nitrogen (N2) and carbon monoxide (CO).
U.S. Pat. No. 4,917,711 discloses the preparation of an adsorbent for selectively adsorbing carbon monoxide using a solid mixture comprising a support selected from among zeolite, alumina, silica gel, aluminosilicate, aluminophosphate and combinations thereof, and a cuprous compound, or further comprising a solvent. In this disclosure, using a solvent selected from among water, an aqueous solution containing hydrochloric acid, primary or secondary alcohols having 1˜7 carbon atoms, acetone, ethylacetate, hydrocarbons having 4˜7 carbon atoms, propionitrile and acetonitrile, a cupric compound supported on the support is converted into the corresponding cuprous compound.
U.S. Pat. No. 5,300,271 discloses an active composition, such as an adsorbent and a catalyst, comprising a cuprous compound dispersed on amorphous oxide or a macroporous carbon support. Such an active composition is prepared by supporting a cupric compound on a pretreated support with the aid of an aqueous solution of ammonium citrate and then subjecting the cupric compound to activation or reduction to the corresponding cuprous compound. Such a cuprous compound is used to separate carbon monoxide or olefins.
US Patent Application Publication No. 2003-0126989 discloses the purification of a CO/H2 or N2/H2 gas mixture containing carbon dioxide (CO2), water (H2O), hydrocarbons, and/or NOx before supplying it to a cryogenic process. This disclosure pertains to a temperature swing adsorption method and/or a pressure swing adsorption method using zeolite NaLSX (a ratio of Si/Al is 0.9˜1.1) to remove such impurities.
US Patent Application Publication No. 2003-0172808 discloses the appropriate arrangement and use of adsorbents, comprising activated carbon, alumina, or silica gel, to remove water (H2O), carbon dioxide (CO2) and hydrocarbons having 1˜8 carbon atoms from a hydrogen-containing gas mixture including impurities such as carbon dioxide (CO2), hydrocarbons having 1˜8 carbon atoms, water (H2O), carbon monoxide (CO) and nitrogen (N2), Ca ion-exchanged zeolite 5A to selectively adsorb and remove carbon monoxide (CO), and Ca or Li ion-exchanged zeolite LSX (low silica X zeolite) to remove nitrogen (N2), thereby increasing the productivity of hydrogen and the yield of purified hydrogen.
U.S. Pat. No. 6,340,382 discloses a PSA method for purifying hydrogen from a hydrogen-containing gas mixture including water (H2O), nitrogen (N2), carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) as impurities. This disclosure pertains to a PSA process using a plurality of adsorption columns which are packed with an alumina bed for removing water (H2O), an activated carbon bed for removing carbon dioxide (CO2), methane (CH4) and carbon monoxide (CO), and an ion-mixed zeolite bed (molar ratio of SiO2/Al2O3 is 2.0˜2.5) containing CaX, LiA, LiX or a Ca cation for adsorbing and removing nitrogen.
U.S. Pat. No. 6,464,756 discloses the purification of hydrogen through a PSA method from a hydrogen-containing gas mixture including carbon monoxide (CO) and/or nitrogen (N2) as impurities. In this disclosure, the adsorbent is exemplified by faujasite zeolite having a molar ratio of Si/Al of 1˜3. Particularly useful is faujasite zeolite having a molar ratio of Si/Al of 1˜1.5 and an aluminum tetrahedral framework in which the proportion of crystal lattices associated with lithium cations and calcium cations is at least 85%, so that the ratio of lithium/(lithium+calcium) is at least 70%.
US Patent Application Publication No. 2005-0257685 discloses a PSA process for producing a less strongly adsorbable component (e.g., hydrogen), in which one process cycle comprising steps of pressurization, pressure equalization, production of predetermined product gas, countercurrent depressurization, and purging is iterated while continuously supplying a gas mixture to be separated, and a plurality of adsorption columns and specific 12 steps are applied.
US Patent Application Publication No. 2006-0236862 discloses a PSA system composed of a plurality of adsorption columns for hydrogen purification and passing reformer exhaust gases, including hydrogen (H2), carbon monoxide (CO), carbon dioxide (CO2), methane (CH4), and nitrogen (N2), through adsorption columns. The respective adsorption columns have a first adsorbent bed for selectively adsorbing carbon monoxide (CO) and nitrogen (N2), and a second adsorbent bed disposed between the inlet of the adsorption column and the first adsorbent bed to selectively adsorb methane (CH4) and carbon dioxide (CO2). The adsorbent charged in the first bed is characterized in that, for carbon monoxide (CO), a Henry's law constant ranges from about 2.5 to about 5.5 mol/g/atm, and the heat of adsorption is between about 6.0 to about 7.5 kcal/gmole, a Henry's law constant for nitrogen (N2) is greater than about 1.5 mol/g/atm, and the selectivity of carbon monoxide (CO) to nitrogen (N2) is about 5.0˜8.0. As the adsorbent material having such performance, an adsorbent composed of a binder and zeolite 5A, which is exchanged with 70˜95% Ca, an adsorbent which is binderless and includes zeolite 5A, which is exchanged with 60˜91% Ca, and a cuprous chloride (CuCl)-impregnated activated carbon adsorbent are claimed. Further, activated carbon is claimed as the adsorbent of the second bed.
US Patent Application Publication No. 2006-0254425 discloses an optimal set of adsorbents for use in hydrogen purification. Each adsorption column is divided into four regions, in which a first region includes an adsorbent for removing water (H2O), a second region includes a mixture of weak and strong adsorbents to remove bulky impurities such as carbon dioxide (CO2), a third region includes a high-density adsorbent for removing remaining carbon dioxide (CO2), most methane (CH4), and carbon monoxide (CO), and a fourth region includes an adsorbent having a high Henry's law constant to remove nitrogen (N2) and residual impurities. This disclosure pertains to a PSA method for producing a highly pure hydrogen product as desired by passing a feed gas through the adsorption column having four regions.
Finally, Korean Patent Application No. 2007-54275, submitted by the present applicant, discloses a solid adsorbent, obtained by dissolving cuprous salts or mixtures thereof in a predetermined solvent to thus prepare a stabilized cuprous salt solution, which is then brought into contact with a solid support so that the cuprous salt is supported and dispersed on the solid support. This disclosure is advantageous because the selectivity to carbon monoxide can be increased, and further, the content of carbon monoxide in the hydrogen-containing gas mixture can be decreased to a very low level.
In this way, in order to remove impurities from the gas mixture containing hydrogen gas, conventionally, the PSA process using activated carbon and zeolite 5A adsorbents is mainly applied. In particular, zeolite 5A is useful as an adsorbent for selectively reversibly adsorbing carbon monoxide.
More specifically, to date, in the PSA process for hydrogen purification, activated carbon is used to remove carbon dioxide (CO2) and methane (CH4), and Ca ion-exchanged zeolite 5A is used to remove methane (CH4), carbon monoxide (CO), or nitrogen (N2).
However, because the zeolite 5A adsorbent has strong ability to adsorb carbon dioxide and water, when carbon dioxide and water are adsorbed, the ability to adsorb carbon monoxide is decreased, undesirably causing problems in which a hydrogen product resulting from the PSA process using the zeolite 5A adsorbent cannot but contain a considerable amount of carbon monoxide. This problem may be solved in such a way that the adsorption column for PSA is packed with an excess of the zeolite 5A adsorbent, but this solution is disadvantageous.
Further, taking into consideration the efficiency of use of purified hydrogen, in a PEMFC (Proton Exchange Membrane Fuel Cell), hydrogen is a reactant on the anode side, and oxygen is a reactant on the cathode side. The anode and the cathode are formed of fine catalyst particles. Such a catalyst is typically precious metal. A membrane electrode assembly is somewhat expensive to manufacture and requires specific conditions for efficient operation. For example, because carbon monoxide (CO), present in the reactants, poisons the platinum catalyst used for the anode and cathode, in order to decrease the load of the catalyst, the content of carbon monoxide in the hydrogen fuel used for the anode should be 10 ppmv or less, and preferably 3 ppmv or less.